Pseudo two-step connection

ABSTRACT

A threaded connection includes a continuous pin thread including at least a first step, a mid-step, and a second step formed sequentially thereon. A continuous box thread includes at least a first step, a mid-step, and a second step formed sequentially thereon, wherein the steps on the box thread correspond generally in axial position with the steps on the pin thread. The first step has a first wedge ratio, the mid-step has a transition wedge ratio, and the second step has a second wedge ratio, wherein thread leads are substantially constant within each of the steps. The connection is designed such that at make-up of a pin member with a box member, a clearance exists between at least one of corresponding load flanks and corresponding stab flanks on at least one of the first step, the mid-step, and the second step.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is related to co-pending United States patentapplications filed concurrently herewith titled “Threads withPerturbations” having U.S. patent application Ser. No. 11/027,014, andtitled “Floating Wedge Thread for Tubular Connection” having U.S. patentapplication Ser. No. 11/027,015, all assigned to the assignee of thepresent application and all incorporated herein by reference in theirentireties.

BACKGROUND OF INVENTION

Casing joints, liners, drill pipe, and drill collars (collectivelyreferred to as “tubulars”) are often used in drilling, completing, andproducing a well. Casing joints, for example, may be emplaced in awellbore to stabilize a formation, to protect a formation againstelevated wellbore pressures (e.g., wellbore pressures that exceed aformation pressure), and the like. Casing joints may be coupled in anend-to-end manner by threaded connections, welded connections, and otherconnections known in the art. The connections may be designed so as toform a seal between an interior of the coupled casing joints and anannular space formed between exterior walls of the casing joints andwalls of the wellbore. The seal may be, for example, an elastomeric seal(e.g. an o-ring seal), a metal-to-metal seal formed proximate theconnection, or similar seals known in the art. In some corrections,seals are formed between the internal and external threads. Connectionswith this characteristic are said to have a “thread seal.” As usedherein, a “thread seal” means that a seal is formed between at least aportion of the internal thread on the box member and the external threadon the pin member.

It will be understood that certain terms are used herein as they wouldbe conventionally understood where tubular joints are being connected ina vertical position along a central axis of the tubular members such aswhen making up a pipe string for lowering into a well bore. Thus, theterm “load flank” designates the side wall surface of a thread thatfaces away from the outer end of the respective pin or box member onwhich the thread is formed and supports the weight (i.e., tensile load)of the lower tubular member hanging in the well bore. The term “stabflank” designates the side wall surface of the thread that faces towardthe outer end of the respective pin or box member and supports forcescompressing the joints toward each other such as the weight of the uppertubular member during the initial makeup of the joint or such as a forceapplied to push a lower tubular member against the bottom of a bore hole(i.e., compressive force). The term “face” of the box is the end of thebox member facing outward from the box threads and the term “nose” ofthe pin is the end of the pin member facing outward from the threads ofthe connection. Upon makeup of a connection the nose of the pin isstabbed into and past the face of the box.

One type of thread commonly used to form a thread seal is a wedgethread. In FIG. 1, a connection having a wedge thread is shown. “Wedgethreads” are characterized by threads that increase in width (i.e.,axial distance between load flanks 125 and 126 and stab flanks 132 and131) in opposite directions on the pin member 101 and box member 102.Wedge threads are extensively disclosed in U.S. Pat. No. RE 30,647issued to Blose, U.S. Pat. No. RE 34,467 issued to Reeves, U.S. Pat. No.4,703,954 issued to Ortloff, and U.S. Pat. No. 5,454,605 issued to Mott,all assigned to the assignee of the present invention and incorporatedherein by reference. On the pin member 101, the pin thread crest 122 isnarrow towards the distal end of the pin member 101 while the box threadcrest 191 is wide. Moving along the axis 105 (from right to left), thepin thread crest 122 widens while the box thread crest 291 narrows. InFIG. 1, the thread surfaces are tapered, meaning that the pin thread 106increases in diameter from beginning to end while the box thread 107decreases in diameter in a complimentary manner. Having a thread taperimproves the ability to stab the pin member 101 into the box member 102and distributes stress in the connection.

Generally, thread seals are difficult to achieve with free-runningthreads having broad crests and roots, however, the same thread formsmay have thread seals when used for wedge threads. Various thread formsmay be used for embodiments of the invention disclosed below. Oneexample of a suitable thread form is a semi-dovetailed thread formdisclosed in U.S. Pat. No. 5,360,239 issued to Klementich, andincorporated herein by reference. Another thread form includes amulti-faceted load flank or stab flank, as disclosed in U.S. Pat. No.6,722,706 issued to Church, and incorporated herein by reference. Anopen thread form with a generally rectangular shape is disclosed in U.S.Pat. No. 6,578,880 issued to Watts. Each of the above thread forms areexample thread forms that may be used for embodiments of the inventionhaving either wedge threads or free running threads. Those havingordinary skill in the art will appreciate that the teachings containedherein are not limited to particular thread forms.

For wedge threads, a thread seal is accomplished by the contact pressurecaused by interference over at least a portion of the connection betweenthe pin load flank 126 and the box load flank 125 and between the pinstab flank 132 and the box stab flank 131, which occurs when theconnection is made-up. Close proximity or interference between the roots192 and 121 and crests 122 and 191 completes the thread seal when itoccurs over at least a portion of where the flank interference occurs.Higher pressure may be contained with increased interference between theroots and crests (“root/crest interference”) on the pin member 101 andthe box member 102 and by increasing flank interference. This particularconnection also includes a metal-to-metal seal that is accomplished bycontact between corresponding sealing surfaces 103 and 104 located onthe pin member 101 and box member 102, respectively.

A property of wedge threads, which typically do not have a positive stoptorque shoulder on the connection, is that the make-up is“indeterminate,” and, as a result, the relative position of the pinmember and box member varies more for a given torque range to be appliedthan connections having a positive stop torque shoulder. As used herein,“make-up” refers to threading a pin member and a box member together.“Selected make-up refers to threading the pin member and the box membertogether with a desired amount of torque, or based on a relativeposition (axial or circumferential) of the pin member with the boxmember. For wedge threads that are designed to have both flankinterference and root/crest interference at a selected make-up, both theflank interference and root/crest interference increase as theconnection is made-up (i.e. increase in torque increases flankinterference and root/crest interference). For wedge threads that aredesigned to have root/crest clearance, the clearance decreases as theconnection is made-up. Regardless of the design of the wedge thread,corresponding flanks and corresponding roots and crests come closer toeach other (i.e. clearance decreases or interference decreases) duringmake-up. Indeterminate make-up allows for the flank interference androot/crest interference to be increased by increasing the torque on theconnection. Thus, a wedge thread may be able to thread seal higherpressures of gas and/or liquid by designing the connection to have moreflank interference and/or root/crest interference or by increasing thetorque on the connection, however, this also increases stress on theconnection during make-up, which could lead to failure during use.

Free-running threads used for oilfield tubular connections typically donot form thread seals when the connection is made-up. FIG. 2 shows aprior art connection having free-running threads. The free-runningthreads include load flanks 154 and 155, stab flanks 157 and 158, crests159 and 162, and roots 160 and 161. As is typical of a connection withfree-running threads, this connection relies on a positive stop torqueshoulder formed by the contact of surfaces 151 and 152 disposed on thepin member 101 and the box member 102, respectively. The positive stoptorque shoulder shown in FIG. 2 is commonly referred to as a “pin noseshoulder.” In other connections, the positive stop torque shoulder mayinstead be formed by the box face 163 and a mating shoulder (not shown)on the pin member 101. The positive stop torque shoulder also provides aseal. Unlike wedge threads, which make-up by the wedging of the pinthread (106 of FIG. 1) and the box thread (107 of FIG. 1), free-runningthreads rely on the positive stop torque shoulder to load the connectionduring make-up. To make-up the connection shown in FIG. 2, the pinmember 101 and the box member 102 are screwed together until thesurfaces 151 and 152 are brought into abutment, at which point the pinload flank 154 and box load flank 155 are also in abutment. Additionaltorque is applied to the pin member 101 and the box member 102 to loadthe surfaces 151 and 152 and the pin load flank 154 and box load flank155 until the desired amount of make-up torque has been applied to theconnection.

The connection shown in FIG. 2 does not accomplish a thread seal becauseof the large gap 153 that exists between the pin stab flank 157 and boxstab flank 158. The gap 153 occurs because of how free-running threadswith positive stop torque shoulders are loaded. Applying torque to theconnection during make-up against the positive stop torque shouldercauses the pin member 101 to be compressed while the box member 102 isstretched in tension. Note that when a box face shoulder is used, thebox member 102 is compressed while the pin member 101 is stretched intension. The force between the pin member 101 and the box member 102 isapplied through the pin load flank 154 and box load flank 155. The pinstab flank 157 and the box stab flank 158 are not loaded during make-up.This results in contact pressure between the load flanks 154 and 155 anda gap between stab flanks 157 and 158. As discussed above, a wedgethread (as shown in FIG. 1) is able to form a thread seal in partbecause of the interference between the load flanks 125 and 126 and thestab flanks 132 and 131. For wedge threads, this occurs near the end ofthe make-up of the connection because of the varying width of the pinthread 106 and the box thread 107. To have similar interference betweenthe load flanks 154 and 155 and stab flanks 157 and 158 on a cylindrical(i.e. non-tapered) free-running thread, the interference would existsubstantially throughout the make-up of the connection because the pinthread 106 and the box thread 107 have a continuous width. Further,root/crest interference, if any, would exist substantially throughoutthe make-up of the connection. This could lead to galling of the threadsand difficulty in making up the connection.

The variance in thread width for a wedge thread occurs as a result ofthe load flanks having different leads than the stab flanks. A threadlead may be quantified in inches per revolution. Note that this is theinverse of a commonly used term “thread pitch,” which is commonlyquantified as threads per inch. A graph of the leads for a prior artwedge thread is shown in FIG. 3A. For this connection, the load lead 14is constant over the length of the connection and greater than the stablead 12, which is also constant. The nominal lead is shown as item 10.As used herein, “nominal lead” refers to the average of the load lead 14and the stab lead 12. The thread will widen with each revolution by thedifference in the load lead 14 and the stab lead 12. The difference inthe load lead 14 and the stab lead 12 is sometimes referred to as the“wedge ratio.” For a free-running thread (i.e. non-wedge thread), theload lead 14 and the stab lead 12 would be substantially equal causingthe free-running thread to have a substantially constant thread width(i.e. a zero wedge ratio).

Intentional variances in thread leads have been disclosed in the priorart for the purposes of load distribution, however, the present inventoris unaware of variances in thread leads to form a thread seal for awedge thread or a free-running thread. One example of a varied threadlead for stress distribution is disclosed in U.S. Pat. No. 4,582,348issued to Dearden, et al. That patent is incorporated herein byreference in its entirety. Dearden discloses a connection withfree-running threads that has the pin thread and box thread divided intothree portions with different leads (note that Dearden refers to threadpitch, which is quantified as threads per inch). In FIG. 3B, a graph ofthe thread leads for the box member and the pin member is shown. Asshown in the graph, at one end of the connection, the pin thread lead 21is larger than the box thread lead 22. In the intermediate portion 23,the pin thread lead 21 and box thread lead 22 are substantially equal.Then, at the other end of the connection, the box thread lead 22 islarger than the pin thread lead 21. In Dearden, the changes in the pinthread lead 21 and box thread lead 22 are step changes (i.e.substantially instantaneous changes in the lead). The varied threadleads disclosed by Dearden are intended to distribute loading across agreater portion of the connection, and have no effect on the inabilityof the free-running threads to form a thread seal. Dearden does notdisclose varying a load lead or stab lead independent of each other.

Another connection is disclosed in U.S. application Ser. No. 10/126,918entitled “Threaded Connection Especially for Radially PlasticallyExpandable Conduit,” (“Sivley”) and assigned to the assignee of thepresent invention. That application is incorporated herein by referencein its entirety. Sivley discloses connections having a variance in loadlead and/or stab lead on one or both of the pin member and the boxmember. A graph of an embodiment disclosed by Sivley is shown in FIG.3C. Sivley discloses varying the load lead 14 relative to the stab lead12 at a selected rate over at least a portion of the pin thread and/orbox thread. In FIG. 3C, the connection is a wedge thread as shown by thedifference between the load lead 14 and the stab lead 12. The load lead14 and the stab lead 12 converge at a linear rate towards the end of thethread. Sivley discloses various other embodiments having load leads 14and stab leads 12 that vary at linear rates relative to each other. Thevariance in the thread leads distributes the loads experienced by theconnection over the length of the connection.

FIG. 9 shows a prior art two-step connection. The threads that form theconnection are separated on two different “steps,” a large stepindicated by the bracket 31 and a small step indicated by the bracket32. The portion between the large step 31 and the small step 32 iscommonly referred to as a mid-step 901. In some connections, themid-step 901 may be used as a metal-to-metal seal. The pin thread creston the small step 32 of the pin member 101, at its full design height,does not interfere with the box thread crest on the large step 31 of thebox member 102 when the pin member 101 is stabbed into the box member102. The diameter of the small step 32 of the pin member 101 is smallerthan the smallest crest-to-crest thread diameter on the large step 31 ofthe box member 102. The pin thread 106 on the small step 32 can bestabbed past the box thread 107 on the large step 31. The threads onboth the small step 32 and the large step 31, which have substantiallythe same nominal lead, engage with each revolution to make-up theconnection. Thus, the number of revolutions during which the threadsslide or rub against each other is reduced for the same number ofengaged threads. A two-step connection allows for each of the steps tohave threads with different characteristics as long there is little orno variance in the nominal lead of the threads on the steps.

A two-step wedge thread connection is disclosed in U.S. Pat. No.6,206,436 issued to Mallis and assigned to the assignee of the presentinvention. That patent is incorporated herein by reference. Mallisdiscloses a two-step wedge thread connection having different wedgeratios, one of which is considered to be an “aggressive” wedge ratio andthe other a “conservative” wedge ratio. “Aggressive” refers to thelarger wedge ratio, and “conservative” refers to the smaller wedgeratio. Everything else the same, the greater the wedge ratio, the moredeterminate the make-up. Too large of a wedge ratio may have aninadequate wedging effect, which can allow the connection to back-offduring use. Smaller wedge ratios are better able to resist backing-offof the connection. Too small of a wedge ratio may have such anindeterminate make-up that galling may occur over the lengthened make-updistance. Mallis discloses that one of the steps can have a wedge ratiothat is optimized for a more determinate make-up (aggressive), while theother step can have a wedge ratio that is optimized for preventingback-off of the connection (conservative).

In U.S. Pat. Nos. 6,174,001 and 6,270,127 issued to Enderle and assignedto the assignee of the present invention, two-step, low torque wedgethreads for tubular connectors are disclosed. Those patents areincorporated herein by reference in their entirety. One of the steps isprovided so that there is interference contact at makeup along at leastone of the complementary stab flanks, load flanks, roots, and crestswhile clearance is provided along another step along at least one of thecomplementary stab flanks, load flanks, roots, and crests, which reducesthe amount of torque required for make-up of the connection whileretaining torque sensitivity, scaling capability, and threads necessaryfor structural purposes.

One problem with two-step connections is that the connection must bethick to reach 100 percent pipe body efficiency. As used herein, “pipebody efficiency” is the tensile strength of the connection relative tothe tensile strength of the tubular. The primary reason for needing athicker connection is the unengaged space of the mid-step, which isrequired so that the threads on the large step can clear the threads onthe small step during stabbing. The mid-step, due to the lack of threadengagement, does not contribute to the overall strength of theconnection. The advantages of having two separate threads often makes upfor the decreased pipe body efficiency, however, it is desirable to havea single step thread that can exhibit the advantages of two-stepconnections.

SUMMARY OF INVENTION

In one aspect, the present invention relates to a threaded connectionhaving wedge threads. The threaded connection includes a pin member anda box member having a continuous pin thread and a continuous box thread,respectively. The pin thread has a pin thread crest, a pin thread root,a pin load flank, and a pin stab flank. The pin thread includes at leasta first step, a mid-step, and a second step formed sequentially thereon.The box thread has a box thread crest, a box thread root, a box loadflank, and a box stab flank. The box thread includes at least a firststep, a mid-step, and a second step formed sequentially thereon. Thesteps on the box thread correspond generally in axial position with thesteps on the pin thread. The first step has a first wedge ratio, themid-step has a transition wedge ratio, and the second step has a secondwedge ratio. The thread leads are substantially constant within each ofthe steps. The connection is designed such that, at make-up of the pinmember with the box member, a clearance exists between at least one ofthe corresponding load flanks and the corresponding stab flanks on atleast one of the first step, the mid-step, and the second step.

In another aspect, the present invention relates to a method ofmanufacturing a threaded connection having wedge threads using a machinetool with a programmable control. The method includes forming a pinmember and a box member having a continuous pin thread and a continuousbox thread, respectively. The pin thread has a pin thread crest, a pinthread root, a pin load flank, and a pin stab flank. The pin threadincludes at least a first step, a mid-step, and a second step formedsequentially thereon. The box thread has a box thread crest, a boxthread root, a box load flank, and a box stab flank. The box threadincludes at least a first step, a mid-step, and a second step formedsequentially thereon. The steps on the box thread correspond generallyin axial position with the steps on the pin thread. The first step has afirst wedge ratio, the mid-step has a transition wedge ratio, and thesecond step has a second wedge ratio. The thread leads are substantiallyconstant within each of the steps. The connection is designed such that,at make-up of the pin member with the box member, a clearance existsbetween at least one of the corresponding load flanks and thecorresponding stab flanks on at least one of the first step, themid-step, and the second step.

In another aspect, the present invention relates to a threadedconnection having wedge threads. The threaded connection includes a pinmember and a box member having a continuous pin thread and a continuousbox thread, respectively. The pin thread has a pin thread crest, a pinthread root, a pin load flank, and a pin stab flank. The pin threadincludes at least a first step, a mid-step, and a second step formedsequentially thereon. The box thread has a box thread crest, a boxthread root, a box load flank, and a box stab flank. The box threadincludes at least a first step, a mid-step, and a second step formedsequentially thereon. The steps on the box thread correspond generallyin axial position with the steps on the pin thread. The first step has afirst wedge ratio, the mid-step has a transition wedge ratio, and thesecond step has a second wedge ratio. The thread leads are substantiallyconstant within each of the steps. The connection is designed such that,at make-up of the pin member with the box member, a clearance existsbetween both the corresponding load flanks and the corresponding stabflanks on at least one of the first step, the mid-step, and the secondstep.

Other aspects and advantages of the invention will be apparent from thefollowing description and the appended claims.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows a cross section of a prior art connection having a wedgethread.

FIG. 2 shows a cross section of a prior art connection having afree-running thread.

FIGS. 3A, 3B, and 3C show graphs of thread leads for prior artconnections.

FIG. 4A shows a schematic representation of a threaded connection inaccordance with one embodiment of the present invention.

FIGS. 4B and 4C show graphs of thread leads versus axial positioncorresponding to the embodiment shown in FIG. 4A.

FIG. 5A shows a schematic representation of a threaded connection inaccordance with one embodiment of the present invention.

FIGS. 5B and 5C show graphs of thread leads versus axial positioncorresponding to the embodiment shown in FIG. 5A.

FIG. 6A shows a schematic representation of a threaded connection inaccordance with one embodiment of the present invention.

FIGS. 6B and 6C show graphs of thread leads versus axial positioncorresponding to the embodiment shown in FIG. 6A.

FIG. 7A shows a schematic representation of a threaded connection inaccordance with one embodiment of the present invention.

FIGS. 7B and 7C show graphs of thread leads versus axial positioncorresponding to the embodiment shown in FIG. 7A.

FIG. 8A shows a schematic representation of a threaded connection inaccordance with one embodiment of the present invention.

FIGS. 8B and 8C show graphs of thread leads versus axial positioncorresponding to the embodiment shown in FIG. 8A.

FIG. 9 shows a cross section of a prior art two-step connection with afree-running thread.

DETAILED DESCRIPTION

The present invention relates to threads for tubular connections. Morespecifically, the present invention relates to threads having two-stepcharacteristics formed on a single thread on a tapered connection.

For the purpose of clarity, several terms are explicitly defined below.As used herein, “a thread lead” refers generally to the group of leadsconsisting of the load lead, the stab lead, and the nominal lead.

As used herein, “helical length” refers to the number of turns of thethread that the contactor is disposed, and may be expressed in thenumber of degrees about the axis of the tubular (i.e. 360 degrees is onethread pitch).

Embodiments of the present invention have variations in wedge ratios ona single thread such that the connection has at least somecharacteristics of a two-step connection. Embodiments of the presentinvention are characterized by at least two distinct portions joined bya transition zone between the two distinct portions. The two distinctportions may be referred to using the same terminology used for two-stepconnections although embodiments of the present invention have a singlestep. In some embodiments, one step may have a different thread height(as measured from root to crest) in order to form a higher pressurethread seal.

Turning to FIGS. 4A-C, a pseudo two-step thread in accordance with oneembodiment of the present invention is shown. FIGS. 4A-C provide anexaggerated example of a pseudo two-step for illustrative purposes. InFIG. 4A, the pin thread 406 that corresponds to the graph in FIG. 4C isshown at a selected make-up with the box thread 407 that corresponds tothe graph in FIG. 4B. In this particular embodiment the pin thread 406and the box thread 407 have been designed to have interference betweenthe load flanks 225 and 226 and the stab flanks 231 and 232 on both afirst step 401 (compare to the small step 32 in FIG. 9) and a secondstep 403 (compare to the large step 31 in FIG. 9), while having aselected clearance between the flanks on the mid-step 402. In oneembodiment, flank interference may occur on one step before the otherstep during make-up, with both the first step 401 and the second step403 having interference at the selected make-up. Further, in oneembodiment, one or both the small step 32 and the large step 31 may haveinterference between only the load flanks or the stab flanks instead ofboth.

To achieve the pseudo two-step configuration shown in FIG. 4A, the loadlead 314 and the stab lead 312 may be varied in a complementary manneron both the pin thread 406 and the box thread 407, as shown in FIGS. 4Band 4C. The nominal lead 310 has been kept substantially constant overthe length of both the pin thread 406 and the box thread 407. Along thefirst step 401, the difference between the load lead 314 and the stablead 310 (i.e. wedge ratio 411) is substantially constant. At the end ofthe first step 401, the wedge ratio 411 increases to wedge ratio 412 byincreasing the load lead 314 by a selected amount while proportionallydecreasing the stab lead 312 such that the nominal lead 310 issubstantially maintained. The wedge ratio 412 is larger than both thewedge ratio 411 on the first step 401 and the wedge ratio 413 on thesecond step 403. This length of the threads at the heightened wedgeratio 412 provides the transition between the first step 401 and thesecond step 403, and may be referred to as the mid-step 402 using theterminology for two-step connections. The mid-step 402 is minor inhelical length compared to the first step 401 and the second step 403.In some embodiments, the helical length of the mid-step 402 may be inincrements of about 360 degrees to prevent eccentric loading of theconnection. After the mid-step 402, the wedge ratio 412 decreases to thewedge ratio 413 on the second step 403, which is about equal to thewedge ratio 411 on the first step 401 in this embodiment.

Continuing with FIGS. 4A-4C, this embodiment has an offset 405 betweenthe mid-step 402 on the pin thread 406 and the box thread 407. Themid-step 402 on the box thread 407 begins at a slightly earlier selectedaxial position than the mid-step 402 on the pin thread 406. This causesthe box thread 407 to “open up” or widen slightly earlier than the pinthread 406, which causes the selected clearance between flanks to occuron the mid-step 402. To return the threads to having flank interferenceon the second step 403, the second step 403 may also begin at an earlierselected axial position on the box thread 407, which allows the pinthread 406 to “catch up” in width. The variations in the load lead 314and the stab lead 312 over the length of the threads allows for theconnection to behave as if it has two separate steps and a mid-step, asin a two-step connection. This allows for a connection to be designed tohave different behavior in each portion of the connection. Those havingordinary skill in the art will appreciate that after using the teachingsof the present disclosure, many combinations of first steps 401,mid-steps 402, and second steps 403 may be achieved using features fortwo-step connections known in the art. Examples of such connections arediscussed below.

In FIGS. 5A-5C, a pseudo two-step thread in accordance with oneembodiment of the present invention is shown. In FIG. 5A, the pin thread406 that corresponds to the graph in FIG. 5C is shown at a selectedmake-up with the box thread 407 that corresponds to the graph in FIG.5B. In this particular embodiment, the pin thread 406 and the box thread407 have been designed to have interference between the load flanks 225and 226 and the stab flanks 231 and 232 on both a first step 401 and asecond step 403, while having a selected clearance between the flanks onthe mid-step 402. The thread shown in FIGS. 5A-5C differs from the oneshown in FIGS. 4A-4C because the wedge ratio 413 of the second step 403is greater than wedge ratio 411 of the first step 401. Two-stepconnections having differential wedge ratios are disclosed in U.S. Pat.No. 6,206,436 issued to Mallis, which was discussed above. Mallis'teachings (including all of the advantages), as they apply to two-stepconnections having two different wedge ratios, are generally applicableto the pseudo two-step connection disclosed herein. Using theterminology from Mallis, in the embodiment shown in FIGS. 5A-5C, thesecond step 403 has the “aggressive” wedge ratio 413, while the firststep 401 has the “conservative” wedge ratio 411.

Turning to FIGS. 6A-6C, another pseudo two-step thread in accordancewith one embodiment of the present invention is shown. In thisparticular embodiment, the pin thread 406 and the box thread 407 havebeen designed to have interference between the load flanks 225 and 226and the stab flanks 231 and 232 on the mid-step, while selectedclearances exist between the flanks on the first step 401 and the secondstep 403. The embodiment shown in FIG. 6A may be desirable for forming athread seal at the mid-step 402. In one embodiment, the mid-step 402 mayalso have increased root/crest interference as disclosed in theconcurrently filed U.S. patent application titled “Threads withPerturbations.” In this particular embodiment, the helical length of themid-step 402, which experiences load before the first step 401 and thesecond step 403, is about 360 degrees in order to prevent eccentricloading. Although FIG. 6A shows the selected clearances between flankson the first step 401 and the second step 403 as about equal, thosehaving ordinary skill in the art will appreciate that, in otherembodiments, the selected clearances may be different. For example, theconnection may be designed such that the second step 403 has a smallerselected clearance than the first step 401. In such an embodiment, thesecond step 403 would be loaded under tension before the first step 401.In other embodiments, the selected clearances may be different betweenload flanks and stab flanks on the same step.

In FIGS. 7A-7C, another pseudo two-step thread in accordance with oneembodiment of the present invention is shown. In this particularembodiment, the pin thread 406 and the box thread 407 have been designedto have interference between the load flanks 225 and 226 and the stabflanks 231 and 232 on the first step 401, while selected clearancesexist between the flanks on the mid-step 402 and the second step 403.FIGS. 7B and 7C show how the pseudo two-step thread in FIG. 7A can beachieved. In this embodiment, the mid-step 402 on the box thread 407 hasan offset 405 from the mid-step 402 on the pin thread 406, which causesthe box thread 407 to open up before the pin thread 406 widens. Thiscauses a selected clearance to occur between the flanks on the mid-step402. To maintain at least some clearance between the flanks on thesecond step 403, the mid-step 402 on the pin thread 406 has a shorterhelical length than the mid-step 402 on the box thread 407 such that itends at about the same axial position as the mid-step 402 on the boxthread 407. The configuration shown in FIG. 7A allows for stresses to bedistributed along the connection based on the amount of stressexperienced by the connection. For example, a pseudo two-step connectionmay be designed to initially load the first step 401 when pulled intension, and then load the second step 403 prior to yielding the threadsin the first step 401. Such a design is disclosed for two-stepconnections in the concurrently filed U.S. patent application titled“Floating Wedge Thread for Tubular Connection.”

Turning to FIGS. 8A-8C, another pseudo two-step thread in accordancewith one embodiment of the present invention is shown. In thisparticular embodiment, the pin thread 406 and the box thread 407 havebeen designed to have alternating interference and clearance between theload flanks 225 and 226 and the stab flanks 231 and 232 on the steps. Indesigning the thread shown in FIG. 8A, the first step 401 is made tohave interference between the stab flanks 231 and 232, while havingclearance between the load flanks 225 and 226. To alternate betweeninterference and clearance, the load lead 314 and the stab lead 312 ofone of the pin thread 406 and the box thread 407 may be offset from eachother in their axial positions. In this embodiment, the pin thread 406has the offset 408. In another embodiment, the offset 408 may be on thebox thread 407.

Continuing with the embodiment shown in FIGS. 8A-8C, the increase in theload lead 314 of the pin thread 406 begins before the increase in theload lead 314 of the box thread 407. This causes the pin thread 406 towiden on the load flank side, which brings the load flanks 225 and 226into interference at the mid-step 402. Then, the decrease in the stablead 312 of the box thread 407 begins before the decrease in the stablead 312 of the pin thread 406, which brings the stab flanks 231 and 232out of interference at the mid-step 402. The flank interference is thenreversed back at the end of the mid-step 402 by decreasing the loadleads 314 and increasing the stab leads 312 with the same offsets inaxial position. This causes the second step 403 to have interferencebetween the stab flanks 231 and 232, while having clearance between theload flanks 225 and 226. In another embodiment, the alternating ofinterference and clearance may be reversed (i.e. having interferencebetween the load flanks 225 and 226 on first step 401, while havingclearance between the stab flanks 231 and 232).

While each of the above embodiments shows at least some clearancebetween flanks, it should be noted that some embodiments of the pseudotwo-step connection may be designed to have varying amounts ofinterference between flanks on each of the first step, the mid-step, andthe second step at a selected make-up. A pseudo two-step connection maybe made such that interference occurs in a sequential manner betweenload flanks and stab flanks on the first step, the mid-step, and thesecond step. For example, by using the offsetting methods of load leadand stab lead changes discussed with respect to the above embodiments, apseudo two-step connection may be designed such that during make-up, theflanks on the second step come into interference. Then as the make-upcontinues, the flanks on the first step come into interference, withflanks on the mid-step coming into interference last. As discussedabove, flank interference increases during make-up of a wedge threadconnection. As a result, at a selected make-up, the step on which flankinterference occurs first will have the most interference. Those havingordinary skill in the art will appreciate that many combinations andsequences of interference and clearance between flanks are possibleusing the teachings of the present invention. Thus, the scope of thepresent invention should not be limited to the select number ofembodiments disclosed herein.

Another variation that is possible is the relative helical lengths ofthe first step, the mid-step, and the second step. While the aboveembodiments have shown first steps that are substantially equal inhelical length to the second steps, those having ordinary skill in theart will appreciate the first step and second step may be unequal inhelical length. For example, on a connection having about 10 threadturns (i.e. about 3600 degrees in helical length), the first step may beabout 4 thread turns (i.e. about 1440 degrees in helical length), whilethe mid-step may be about 1 thread turn and the second step may be about5 thread turns.

It should be noted that the graphs of thread leads for the aboveembodiments are idealized as step changes in the thread leads. Inpractice, the changes in the thread leads may not be as instantaneous asshown in the graphs due to the manufacturing process used to make thethreads. For example, in one embodiment, a computer numericallycontrolled (“CNC”) lathe may be used. CNC machines may be controlled byCNC programs. Typically, the CNC program consists of positions for eachaxis of control. For example, if the CNC lathe has an axial position anda rotational position, the program would have an axial position valuecorresponding with each rotational position. Because a CNC lathe isusually rotating at a set speed measured in rotations per minute(“RPM”), the CNC program typically has the rotational positions in orderand at set increments as the part is rotated in the machine. Theincrements at which the rotational positions are spaced is commonlyreferred to as the “resolution” of the lathe. For example, if theresolution is about 90 degrees, a data point will exist for eachsequential increment of about 90 degrees. An axial position would beselected for each increment. Typically, the CNC lathe will move theaxial position at a substantially constant speed between points. Thespeed is selected as required to reach the next axial position atsubstantially the same time as the corresponding rotational position.The thread lead can be selected by calculating the value for theincrements such that for each revolution, the axial position advances bya distance substantially equal to the thread lead. For example, a leadof 1 inch per revolution would advance by a ¼ inch every 90 degrees.Those having ordinary skill in the art will be able to apply the aboveteachings for use with other manufacturing methods. The resolution ofthe lathe used may effect the amount of offset between steps. Anotherresult of using machine tools is that the momentum of the moving partsand response time in the controls may result in a more smoothed outchange in thread leads. Although the precise changes in thread leadsbetween the first step, the mid-step, and the second step may vary byproduction method, the benefits of the pseudo two-step connection maystill be realized.

It should be noted that embodiments of the present invention have atleast a first step and a second step with a transition zone (i.e.mid-step) joining the first step and the second step. The first step,the mid-step, and the second step are formed sequentially on both thepin thread and the box thread. Those having ordinary skill in the artwill appreciate that additional steps may be added to the pin thread andthe box thread without departing from the scope of the presentinvention. Further, embodiments of the present invention may be formedon an actual two-step connection. For example, a pseudo two-step inaccordance with the above disclosure may be formed on one of the smallstep and the large step of a two-step connection such that theconnection essentially has three steps.

While the invention has been described with respect to a limited numberof embodiments, those skilled in the art, having benefit of thisdisclosure, will appreciate that other embodiments can be devised whichdo not depart from the scope of the invention as disclosed herein.Accordingly, the scope of the invention should be limited only by theattached claims.

1. A threaded connection having wedge threads, the threaded connection comprising: a pin member comprising a continuous pin thread having a pin thread crest, a pin thread root, a pin load flank, and a pin stab flank, wherein the continuous pin thread comprises at least a first step, a mid-step, and a second step formed sequentially thereon; a box member comprising a continuous box thread having a box thread crest, a box thread root a box load flank, and a box stab flank, wherein the continuous box thread comprises at least a first step, a mid-step, and a second step formed sequentially thereon, the steps on the continuous box thread corresponding generally in axial position with the steps on the pin thread, wherein the first step has a first wedge ratio, the mid-step has a transition wedge ratio, and the second step has a second wedge ratio, wherein thread leads are substantially constant within each of the steps, wherein the connection is designed such that at make-up of the pin member with the box member, a clearance exists between at least one of the corresponding load flanks and the corresponding stab flanks on at least one of the first step, the mid-step, and the second step.
 2. The threaded connection of claim 1, wherein a beginning of the mid-step on the continuous pin thread is offset in axial position from a beginning of the mid-step on the box thread.
 3. The threaded connection of claim 1, wherein a selected clearance exists between both the corresponding load flanks and the corresponding stab flanks on the mid-step.
 4. The threaded connection of claim 1, wherein a selected clearance exists between both the corresponding load flanks and the corresponding stab flanks on both the first step and the second step.
 5. The threaded connection of claim 1, wherein one of the first wedge ratio and the second wedge ratio is greater than the other.
 6. The threaded connection of claim 1, wherein the first wedge ratio and the second wedge ratio are substantially equal.
 7. The threaded connection of claim 1, wherein one of the continuous pin thread and the continuous box thread has a greater height on one of the first step, the mid-step, and the second step.
 8. The threaded connection of claim 1, wherein during make-up of the pin member with the box member, interference between at least one of the corresponding load flanks and the corresponding stab flanks occurs sequentially on the first step, the mid-step, and the second step.
 9. A threaded connection having wedge threads, the threaded connection comprising: a pin member comprising a continuous pin thread having a pin thread crest, a pin thread root, a pin load flank, and a pin stab flank, wherein the continuous pin thread comprises at least a first step, a mid-step, and a second step formed sequentially thereon; a box member comprising a continuous box thread having a box thread crest, a box thread root, a box load flank, and a box stab flank, wherein the continuous box thread comprises at least a first step, a mid-step, and a second step formed sequentially thereon, the steps on the continuous box thread corresponding generally in axial position with the steps on the continuous pin thread, wherein the first step has a first wedge ratio, the mid-step has a transition wedge ratio, and the second step has a second wedge ratio, wherein thread leads are substantially constant within each of the steps, wherein the connection is designed such that at make-up of the pin member with the box member, a clearance exists between both the corresponding load flanks and the corresponding stab flanks on at least one of the first step, the mid-step, and the second step.
 10. The threaded connection of claim 9, wherein a beginning of the mid-step on the continuous pin thread is offset in axial position from a beginning of the mid-step on the box thread.
 11. The threaded connection of claim 9, wherein a selected clearance exists between both the corresponding load flanks and the corresponding stab flanks on the mid-step.
 12. The threaded connection of claim 9, wherein a selected clearance exists between both the corresponding load flanks and the corresponding stab flanks on both the first step and the second step.
 13. The threaded connection of claim 9, wherein one of the first wedge ratio and the second wedge ratio is greater than the other.
 14. The threaded connection of claim 9, wherein the first wedge ratio and the second wedge ratio are substantially equal.
 15. The threaded connection of claim 9, wherein one of the continuous pin thread and the continuous box thread has a greater height on one of the first step, the mid-step, and the second step.
 16. The threaded connection of claim 9, wherein during make-up of the pin member with the box member, interference between at least one of the corresponding load flanks and the corresponding stab flanks occurs sequentially on the first step, the mid-step, and the second step. 